Androgen deprivation therapy (ADT) is initially effective in treating metastatic prostate cancer, and secondary hormonal therapies are being tested to suppress androgen receptor (AR) reactivation in castration-resistant prostate cancer (CRPC). Despite variable responses to AR pathway inhibitors in CRPC, there are no reliable biomarkers to guide their application. Here, we used microfluidic capture of circulating tumor cells (CTC) to measure AR signaling readouts before and after therapeutic interventions. Single-cell immunofluorescence analysis revealed predominantly “AR-on” CTC signatures in untreated patients, compared with heterogeneous (“AR-on, AR-off, and AR-mixed”) CTC populations in patients with CRPC. Initiation of first-line ADT induced a profound switch from “AR-on” to “AR-off” CTCs, whereas secondary hormonal therapy in CRPC resulted in variable responses. Presence of “AR-mixed” CTCs and increasing “AR-on” cells despite treatment with abiraterone acetate were associated with an adverse treatment outcome. Measuring treatment-induced signaling responses within CTCs may help guide therapy in prostate cancer.

Significance: Acquired resistance to first-line hormonal therapy in prostate cancer is heterogeneous in the extent of AR pathway reactivation. Measurement of pre- and posttreatment AR signaling within CTCs may help target such treatments to patients most likely to respond to second-line therapies. Cancer Discov; 2(11); 995–1003. ©2012 AACR.

Read the Commentary on this article by Pantel and Alix-Panabières, p. 974.

This article is highlighted in the In This Issue feature, p. 961